Intraseasonal modulation of Indian summer monsoon by Middle East dust : an observational and numerical modeling study

textAs one of the world's strongest monsoon systems, the Indian summer monsoon affects one-third of the global population. The temporal and spatial variability of the ISM rainfall has great socio-economic impacts, particularly on agriculture and food supply in South Asia. Additionally, South Asia is an area with heavy atmospheric loading of aerosols from both wind-blown mineral dust and manmade pollutants. These aerosols can significantly influence the ISM rainfall through their radiative and microphysical effects. In our study, we focus on three questions. 1) How does the ISM rainfall respond to the Middle East dust in observations on intraseasonal timescales? 2) Can the regional climate model reproduce the observed relationship between Middle East dust and the ISM rainfall, and what are the model uncertainties and how do they influence our results? 3) How does the ISM system respond to different types of aerosols in different source regions on the intraseasonal timescales? I use multiple satellite retrievals, reanalysis datasets, and ensemble modeling experiments to study the dust-monsoon connection. Multivariate empirical orthogonal function is performed on aerosol optical depth and rainfall from satellite observations, and winds and geopotential height from reanalysis to identify the coupled spatial patterns among these variables. Cross-correlation analyses between satellite-retrieved AOD in the Middle East and the ISM rainfall are calculated to characterize the timescales of dust-monsoon connections. Furthermore, ensemble numerical experiments are conducted to examine the causal relationship and physical mechanisms between Middle East dust aerosols and the ISM monsoon. The ensemble experiments are created by perturbing physical and chemical model schemes to examine the uncertainties in parameterizing the shortwave radiation, dust diffusion in the boundary layer, and aerosol chemical mixing rules. The primary scientific findings are summarized here. (1) Middle East dust aerosols are positively correlated with the ISM rainfall in Pakistan, central and northern India, and Coastal South West India. (2) The timescale of the dust–monsoon connection is about 11 to 13 days. (3) Middle East dust aerosols can enhance the southwesterly monsoon flow over the Arabian Sea due to their direct radiative heating effect in the lower troposphere, which can increase the south–north ocean–land thermal contrast. The enhanced monsoon flow can transport more water vapor from the Arabian Sea to the Indian subcontinent, thereby resulting in more monsoon rainfall. (4) Middle East dust aerosols play a dominant role in modulating the ISM rainfall compared to dust aerosols from other regions; local anthropogenic aerosols in India, although with much lower concentrations than dust, can play a similar role to Middle East dust aerosols. Our findings demonstrate that a better representation of Middle East dust aerosols and their interactions with meteorological fields is important for understanding and modeling the variability of the ISM rainfall.Geological Science

Irrigation-Induced Environmental Changes around the Aral Sea: An Integrated View from Multiple Satellite Observations

The Aral Sea basin (ASB) is one of the most environmentally vulnerable regions to climate change and human activities. During the past 60 years, irrigation has greatly changed the water distribution and caused severe environmental issues in the ASB. Using remote sensing data, this study investigated the environmental changes induced by irrigation activities in this region. The results show that, in the past decade, land water storage has significantly increased in the irrigated upstream regions (13 km 3 year -1 ) but decreased in the downstream regions (-27 km 3 year -1 ) of the Amu Darya River basin, causing a water storage decrease in the whole basin (-20 km 3 year -1 ). As a result, the water surface area of the Aral Sea has decreased from 32,000 in 2000 to 10,000 km2 in 2015. The shrinking Aral Sea exposed a large portion of the lake bottom to the air, increasing (decreasing) the daytime (nighttime) temperatures by about 1 °C year -1 (0.5 °C year -1 ). Moreover, there were other potential environmental changes, including drier soil, less vegetation, decreasing cloud and precipitation, and more severe and frequent dust storms. Possible biases in the remote sensing data due to the neglect of the shrinking water surface area of the Aral Sea were identified. These findings highlight the severe environmental threats caused by irrigation in Central Asia and call attention to sustainable water use in such dryland regions. Keywords: environmental issues; the shrinking Aral Sea; irrigation; desertification; dust storm; remote sensing; NDVI; GRACE; MODI

The greening of Northwest Indian subcontinent and reduction of dust abundance resulting from Indian summer monsoon revival

The trends of both rainfall and circulation strength of the Indian summer monsoon has been reviving since 2002. Here, using observational data, we demonstrate a statistically significant greening over the Northwest Indian Subcontinent and a consequent decline in dust abundance due to the monsoon revival. The enhanced monsoonal rainfall causes an increase in soil moisture, which results in a significant greening in the Northwest Indian Subcontinent. These increases in rainfall, soil moisture, and vegetation together lead to a substantial reduction of the dust abundance in this region, especially the Thar Desert, as shown by a negative trend in satellite-retrieved aerosol optical depth. The monsoonal rainfall-induced trends in vegetation growth and dust abundance in the Northwest Indian Subcontinent have important implications for agriculture production and air quality given the projected increases and a westward expansion of the global summer monsoon rainfall at the end of this century.National Science Foundation (U.S.) (Grant AGS-1339264)United States. Department of Energy (Award DE-FG02-94ER61937

Irrigation-Induced Environmental Changes around the Aral Sea: An Integrated View from Multiple Satellite Observations

The Aral Sea basin (ASB) is one of the most environmentally vulnerable regions to climate change and human activities. During the past 60 years, irrigation has greatly changed the water distribution and caused severe environmental issues in the ASB. Using remote sensing data, this study investigated the environmental changes induced by irrigation activities in this region. The results show that, in the past decade, land water storage has significantly increased in the irrigated upstream regions (13 km3 year−1) but decreased in the downstream regions (−27 km3 year−1) of the Amu Darya River basin, causing a water storage decrease in the whole basin (−20 km3 year−1). As a result, the water surface area of the Aral Sea has decreased from 32,000 in 2000 to 10,000 km2 in 2015. The shrinking Aral Sea exposed a large portion of the lake bottom to the air, increasing (decreasing) the daytime (nighttime) temperatures by about 1 °C year−1 (0.5 °C year−1). Moreover, there were other potential environmental changes, including drier soil, less vegetation, decreasing cloud and precipitation, and more severe and frequent dust storms. Possible biases in the remote sensing data due to the neglect of the shrinking water surface area of the Aral Sea were identified. These findings highlight the severe environmental threats caused by irrigation in Central Asia and call attention to sustainable water use in such dryland regions

Impacts on cloud radiative effects induced by coexisting aerosols converted from international shipping and maritime DMS emissions

International shipping emissions (ISE), particularly sulfur dioxide, can influence the global radiation budget by interacting with clouds and radiation after being oxidized into sulfate aerosols. A better understanding of the uncertainties in estimating the cloud radiative effects (CREs) of ISE is of great importance in climate science. Many international shipping tracks cover oceans with substantial natural dimethyl sulfide (DMS) emissions. The interplay between these two major aerosol sources on CREs over vast oceanic regions with a relatively low aerosol concentration is an intriguing yet poorly addressed issue confounding estimation of the CREs of ISE. Using an Earth system model including two aerosol modules with different aerosol mixing configurations, we derive a significant global net CRE of ISE (−0.153 W m−2 with a standard error of ±0.004 W m−2) when using emissions consistent with current ship emission regulations. This global net CRE would become much weaker and actually insignificant (−0.001 W m−2 standard error of ±0.007 W m−2) if a more stringent regulation were adopted. We then reveal that the ISE-induced CRE would achieve a significant enhancement when a lower DMS emission is prescribed in the simulations, owing to the sublinear relationship between aerosol concentration and cloud response. In addition, this study also demonstrates that the representation of certain aerosol processes, such as mixing states, can influence the magnitude and pattern of the ISE-induced CRE. These findings suggest a reevaluation of the ISE-induced CRE with consideration of DMS variability

Modeling dust sources, transport, and radiative effects at different altitudes over the Tibetan Plateau

Mineral dust plays an important role in the climate of the Tibetan Plateau (TP) by modifying the radiation budget, cloud macro- and microphysics, precipitation, and snow albedo. Meanwhile, the TP, with the highest topography in the world, can affect intercontinental transport of dust plumes and induce typical distribution characteristics of dust at different altitudes. In this study, we conduct a quasi-global simulation to investigate the characteristics of dust source contribution and transport over the TP at different altitudes by using a fully coupled meteorology–chemistry model, the Weather Research and Forecasting model with chemistry (WRF-Chem), with a tracer-tagging technique. Generally, the simulation reasonably captures the spatial distribution of satellite-retrieved dust aerosol optical depth (AOD) at different altitudes. Model results show that dust particles are emitted into atmosphere through updrafts over major desert regions and then transported to the TP. The East Asian dust (mainly from the Gobi and Taklamakan deserts) is transported southward and is lifted up to the TP, contributing a mass loading of 50 mg m−2 at a height of 3 km and 5 mg m−2 at a height of 12 km over the northern slope of the TP. Dust from North Africa and the Middle East are concentrated over both of the northern and southern slopes below 6 km, where mass loadings range from 10 to 100 and 1 to 10 mg m−2 below 3 km and above 9 km, respectively. As the dust is transported to the north and over the TP, mass loadings are 5–10 mg m−2 above a height of 6 km. The dust mass flux carried from East Asia to the TP is 7.9 Tg yr−1, mostly occurring at heights of 3–6 km. The dust particles from North Africa and the Middle East are transported eastward following the westerly jet and then are carried into the TP at the west side with dust mass fluxes of 7.8 and 26.6 Tg yr−1, respectively. The maximum mass flux of the North African dust mainly occurs at 0–3 km (3.9 Tg yr−1), while the Middle Eastern dust occurs at 6–9 km (12.3 Tg yr−1). The dust outflow occurs on the east side (−17.89 Tg yr−1) and south side (−11.22 Tg yr−1) of the TP, with a peak value (8.7 Tg yr−1) at 6–9 km. Moreover, the dust (by mass) is concentrated within the size range of 1.25–5.0 µm and the dust (by particle number) is concentrated in the size range of 0.156–1.25 µm. Compared with other aerosols, the dust contributes to more than 50 % of the total AOD over the TP. The direct radiative forcing induced by the dust is −1.28 W m−2 at the top of the atmosphere (cooling), 0.41 W m−2 in the atmosphere (warming), and −1.68 W m−2 at the surface (cooling). Our quantitative analyses of the dust contributions from different source regions and the associated radiative forcing can help us to better understand the role of dust on the climate over the TP and surrounding regions

Effective radiative forcing in the aerosol–climate model CAM5.3-MARC-ARG

Abstract We quantify the effective radiative forcing (ERF) of anthropogenic aerosols modelled by the aerosol–climate model CAM5.3-MARC-ARG. CAM5.3-MARC-ARG is a new configuration of the Community Atmosphere Model version 5.3 (CAM5.3) in which the default aerosol module has been replaced by the two-Moment, Multi-Modal, Mixing-state-resolving Aerosol model for Research of Climate (MARC). CAM5.3-MARC-ARG uses the ARG aerosol-activation scheme, consistent with the default configuration of CAM5.3. We compute differences between simulations using year-1850 aerosol emissions and simulations using year-2000 aerosol emissions in order to assess the radiative effects of anthropogenic aerosols. We compare the aerosol lifetimes, aerosol column burdens, cloud properties, and radiative effects produced by CAM5.3-MARC-ARG with those produced by the default configuration of CAM5.3, which uses the modal aerosol module with three log-normal modes (MAM3), and a configuration using the modal aerosol module with seven log-normal modes (MAM7). Compared with MAM3 and MAM7, we find that MARC produces stronger cooling via the direct radiative effect, the shortwave cloud radiative effect, and the surface albedo radiative effect; similarly, MARC produces stronger warming via the longwave cloud radiative effect. Overall, MARC produces a global mean net ERF of −1.79±0.03 W m−2, which is stronger than the global mean net ERF of −1.57±0.04 W m−2 produced by MAM3 and −1.53±0.04 W m−2 produced by MAM7. The regional distribution of ERF also differs between MARC and MAM3, largely due to differences in the regional distribution of the shortwave cloud radiative effect. We conclude that the specific representation of aerosols in global climate models, including aerosol mixing state, has important implications for climate modelling

Global warming in the pipeline

Improved knowledge of glacial-to-interglacial global temperature change implies that fast-feedback equilibrium climate sensitivity is at least ~4{\deg}C for doubled CO2 (2xCO2), with likely range 3.5-5.5{\deg}C. Greenhouse gas (GHG) climate forcing is 4.1 W/m2 larger in 2021 than in 1750, equivalent to 2xCO2 forcing. Global warming in the pipeline is greater than prior estimates. Eventual global warming due to today's GHG forcing alone -- after slow feedbacks operate -- is about 10{\deg}C. Human-made aerosols are a major climate forcing, mainly via their effect on clouds. We infer from paleoclimate data that aerosol cooling offset GHG warming for several millennia as civilization developed. A hinge-point in global warming occurred in 1970 as increased GHG warming outpaced aerosol cooling, leading to global warming of 0.18{\deg}C per decade. Aerosol cooling is larger than estimated in the current IPCC report, but it has declined since 2010 because of aerosol reductions in China and shipping. Without unprecedented global actions to reduce GHG growth, 2010 could be another hinge point, with global warming in following decades 50-100% greater than in the prior 40 years. The enormity of consequences of warming in the pipeline demands a new approach addressing legacy and future emissions. The essential requirement to "save" young people and future generations is return to Holocene-level global temperature. Three urgently required actions are: 1) a global increasing price on GHG emissions, 2) purposeful intervention to rapidly phase down present massive geoengineering of Earth's climate, and 3) renewed East-West cooperation in a way that accommodates developing world needs.Comment: 48 pages, 27 figures. Correction of formatting error on page 21, which messed up placement of all following figure

Irrigation-Induced Environmental Changes around the Aral Sea: An Integrated View from Multiple Satellite Observations

The Aral Sea basin (ASB) is one of the most environmentally vulnerable regions to climate change and human activities. During the past 60 years, irrigation has greatly changed the water distribution and caused severe environmental issues in the ASB. Using remote sensing data, this study investigated the environmental changes induced by irrigation activities in this region. The results show that, in the past decade, land water storage has significantly increased in the irrigated upstream regions (13 km3 year−1) but decreased in the downstream regions (−27 km3 year−1) of the Amu Darya River basin, causing a water storage decrease in the whole basin (−20 km3 year−1). As a result, the water surface area of the Aral Sea has decreased from 32,000 in 2000 to 10,000 km2 in 2015. The shrinking Aral Sea exposed a large portion of the lake bottom to the air, increasing (decreasing) the daytime (nighttime) temperatures by about 1 °C year−1 (0.5 °C year−1). Moreover, there were other potential environmental changes, including drier soil, less vegetation, decreasing cloud and precipitation, and more severe and frequent dust storms. Possible biases in the remote sensing data due to the neglect of the shrinking water surface area of the Aral Sea were identified. These findings highlight the severe environmental threats caused by irrigation in Central Asia and call attention to sustainable water use in such dryland regions